SOFC stack systems are well known. An SOFC typically is fueled by reformate gas, which is the partially oxidized effluent from a catalytic partial oxidation (CPOx) hydrocarbon reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen (H2). Because an SOFC stack system is less than 100% fuel efficient, the spent tail gas from the anodes of the stack, also known as anode tail gas or syngas, contains substantial amounts of hydrogen and carbon monoxide, as well as water vapor. To improve stack power density and system efficiency and to reduce carbon precipitation and deposition in the system, it is known in the art to recycle a portion of the tail gas from the stack anodes into either the inlet to the reformer or the inlet to the stack.
In the fuel cell prior art, there is no simple and inexpensive means disclosed for determining accurately the instantaneous mass flow rate of tail gas, which flow rate is necessary in order to control the flow rate to a predetermined control setpoint.
It is known to measure gas flow rates by various means, for example, by turbine meters or by hot wire anemometers. A problem with simply using the output signal of such a gas flow meter is that the molecular composition of tail gas may vary widely over the operating range of a fuel cell stack. Such known flow meters are sensitive to variations in composition of the gas and hence are not directly reliable.
What is needed in the art is a system for calibrating a gas flowmeter to provide accurate measurement and control of anode tail gas recycle flow rates over a wide range of SOFC system operating conditions and recycle mass flow rates.
It is a principal object of the present invention to improve the fuel efficiency of a solid oxide fuel cell stack.